1. Field of the Invention
The present invention relates to methods of joining components, and, in particular, to methods of joining components by laser soldering.
2. Description of Related Art
In laser soldering techniques, a solder material is interposed between surfaces of two components to be joined and is melted by heat from a laser beam. If the solder material has a lower melting point than the components to be joined and the surfaces of the components are such that the molten solder can wet them, a good metallurgical bond is formed as the solder solidifies thus joining the components together.
The formation of a good metallurgical bond is not possible if oxide layers or contaminants on the surfaces to be joined prevent the molten solder from wetting those surfaces.
Additionally, the solder material often has a high reflectivity when solid so that heat transfer from a laser beam directed onto the solder takes place slowly.
The use of flux applied to the surfaces to be joined overcomes these problems since the flux removes oxide layers and contaminants and also has a much lower reflectivity than the solid solder material so that laser energy is transferred quickly to the flux and the solder is melted by conduction.
However, the flux residues remaining after the flux has been melted themselves give rise to contamination around the joint between the components. This is a problem where a highly reliable clean joint is required.
One solution to this problem is to coat the components to be joined with a material which is readily wetted by solder. Noble metals, such as gold, are suitable for this purpose since they do not oxidise in air so that flux is not required to remove any oxide layers. However, where laser soldering techniques are to be used, coupling of laser energy to gold is erratic and difficult to control. In addition, when the gold is wetted by solder, wetting continues to take place until all the solder is consumed, or the temperature drops too low to maintain a fluid metal.
Additionally, if the solder spreads beyond the area of the joint, this results in a weakened joint.